Some telecommunications service providers, including cellular and paging companies, provide a Short Message Service (SMS) for exchanging short alphanumeric messages between a mobile station and the wireless system, and between the wireless system and an external device able to transmit and, optionally, to receive such messages. Examples of such an external device are: a voice telephone, a data terminal, and a short message entry system.
For example, certain paging systems in current use can transmit, in a displayable message, a telephone number that the user is requested to call. Some wireless systems can transmit text for display on the screen of a wireless terminal. There also exist systems that can transmit alphanumeric text to computing devices such as desktop and laptop computers.
The Telecommunications Industry Association (CIA) has prescribed interim standards for transmitting short displayable messages over various wireless air interfaces and networks. Each of these standards is identified by the designation "IS-xxx", wherein "xxx" is a reference number. Each interim standard specifies a protocol, including operations, parameters, operational messages, and procedures for transmitting these short messages. For example, IS-95A, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System," and IS-637, "Short Message Services for Wideband Spread Spectrum Cellular Systems," specify protocols for wireless systems that use Code Division Multiple Access (CDMA) technology.
The standard IS-637 defines a set of technical requirements for a SMS to deliver textual and numeric information for paging, messaging, and voice mail notification. The standards prescribed in IS-637 apply to mobile stations that operate in the analog mode, as well as to those that operate in the wideband spread spectrum CDMA mode. Because it is useful for an understanding of modes of operation of the present invention, IS-637 is hereby incorporated by reference.
Certain wireless communication systems, including at least some CDMA cellular and PCS systems, are designed in such a way that short messages can be delivered to mobile stations in a broadcast fashion. For example, CDMA systems have a common forward (downlink) control channel, also referred to as the Paging Channel. The Paging Channel is normally used for paging, for carrying system overhead information, and for control of mobile stations. However, this channel is also useful for delivery of short messages.
Any mobile station in the idle state, if it has selected a serving system in its locality area and is locked onto the Paging Channel, is continually processing all information that it is receiving from this channel.
Whereas information is often directed to specific mobile stations by setting particular addresses as part of transmitted messages, information of the overhead type is processed by all mobile stations. One messaging mechanism used to deliver information packages containing Short Message Service (SMS) information is Data Burst Message. This messaging mechanism is described in IS-95A, which is hereby incorporated by reference. One specific value of Address Type as a part of the Data Burst Message is the Broadcast Address. By specifying this address, the user can readily direct the message to every mobile station in the relevant coverage area that is on the Paging Channel and currently in the idle state.
Practitioners in the art of wireless communications have recognized a need to make mobile stations use electrical power more efficiently. For this purpose, so-called slotted mode reception has been implemented in wireless systems. In this mode, the mobile station receives information from the Paging Channel intermittently. The infrastructure directs messages to a given mobile station only during time intervals (slots) allocated to the particular group of mobile stations to which the given station belongs. The repetitive cycle of these time slots is defined as the Maximum Paging Slot Cycle. A parameter, referred to as the Slot Cycle Index, specifies this cycle. The Slot Cycle Index is transmitted in the overhead portion of the Paging Channel information block.
The architecture for the conventional use of the Paging Channel is described in the TIA standard IS-95A. One of the system overhead messages conventionally carried in the Paging Channel is the System Parameters Message. This message includes the index parameter which determines the length of the Paging Cycle.
With reference to FIG. 1, each mobile station needs to be awake (with respect to receiving information from the Paging Channel) only during its assigned active slot 10, plus the next adjacent slot 20, according to current practices. During the remainder of the Paging Slot Cycle, the mobile station may subsist in a power-conservation mode.
One way to define an active slot for a given mobile station is to calculate it from the mobile station ID. For example, the mobile station and the base station can readily use a common hashing function to calculate an active slot number from the mobile station ID. Hashing is particularly useful because it assures a relatively even load distribution among different slots on the Paging Channel. Hashing functions useful for this purpose are described in IS-95A.
A Broadcast SMS (B-SMS) message is readily sent to all mobile stations within the relevant coverage area by transmitting it in every paging slot. Thus each mobile station, when it awakens for its assigned paging slot, also receives the B-SMS message. However, this simple mechanism, which involves multiple repetitions of the same B-SMS message in every slot of the cycle, will quickly overload the paging channel and deny slot time to other messages that would otherwise be transmitted. Therefore, it would undesirably reduce the effective capacity of the Paging Channel.